use none as filter value for registry deselection

[andersy005]

• closes Consistent handling of "unexpected" filtering #46

@katamartin, the simplest approach was to enable none as a filter value. let me know if this fulfills the requirements of the front-end.

❯ http 'https://offsets-db-staging.fly.dev/projects/?registry=none'                                                                                                                                           ─╯
HTTP/1.1 200 OK
content-encoding: gzip
content-type: application/json
date: Tue, 31 Oct 2023 14:28:32 GMT
fly-request-id: 01HE325PW34MYXQ3K8VGSPYQJW-lax
server: Fly/7328d5b5 (2023-10-27)
transfer-encoding: chunked
via: 1.1 fly.io

{
    "data": [],
    "pagination": {
        "current_page": 1,
        "next_page": null,
        "total_entries": 0,
        "total_pages": 0
    }
}

[katamartin]

Working perfectly, thanks @andersy005 !